Okay guys, I finally figured it out! This is similar to what
EulerStalker suggested I do, but for some reason it didn't work with 1.2.7, so I clean-installed ver 1.2.8 and it worked. Details below, hope this helps someone else!

If you purchase
Clip Studio Paint English Edition (version 1.3.1) some of the brushes from Japanese Edition ver 1.2.7/1.2.8 are missing. Here's how I got those back:
1. I installed
Clip Studio Paint EX ver. 1.2.8 Japanese Edition.
2. Applied Trev's 1.2.7 translation, which can be found here -->
http://www.mediafire.com/download/nt1v2l3dzgy2sff/CSP-1.2.7_ENG-4.3.rar.
3. Installed
Materials.
4. Installed
Clip Studio Paint EX Ver. 1.3.1 English Edition.
5. Installed
Materials.
6. Navigated to
Documents>CELSYS>CLIPStudioCommon folder. Backed up original files.
7. Navigated to
Documents>CELSYS_EN>CLIPStudioCommon folder. Copied this folder and replaced the original folder located in Step 6.
SIDE NOTE: Doing this translated the Materials section in the Japanese Edition into English, so it seems these two folders are interchangeable.8. Navigated to
CELSYS>CLIPStudioPaintVer1_1_0>Tool folder. Copied both of these files:
default.tgm and
EditImageTool.todb.
9. Navigated to
CELSYS_EN>CLIPStudioPaintVer1_1_0>Tool folder. Pasted both of these files:
default.tgm and
EditImageTool.todb.
10. Opened Clip Studio Paint EX Ver 1.3.1 English Edition.
Missing tools back! Also, Trev's translations replaced English Edition translations for many of the tools (except of course the new ones in ver 1.3.1), although it appears the sub-menus and other menus are still in English Edition.
WHEW!!! Thanks again guys for all your help on this! Your suggestions helped me troubleshoot long enough to finally get a resolution.
